How Reefs Are Built

Coral reefs are built by tiny polyps secreting calcium carbonate skeletons over centuries. A single reef can take thousands of years to grow, creating three-dimensional habitats of extraordinary complexity. The Great Barrier Reef stretches over 2,300 kilometers.

The partnership between coral polyps and their algae is what powers this. Zooxanthellae provide up to ninety percent of a coral's energy through photosynthesis, while the coral offers shelter and nutrients in return.